JPH03138544A - Device for inspecting optical fiber - Google Patents

Abstract

PURPOSE: To improve the accuracy and stability of an inspection by modulating the amplitude of rays from a fiber to be inspected, modulating it again, setting the light path between both modulations to equal to the integer multiples of modulation wavelength, and calculating it from a modulation frequency that is determined by a detector. CONSTITUTION: The device has a first polarization beam splitter 3, an electrooptical modulator crystal 4, and a second polarization ray splitter 7. Then, parallel rays 1 from a light source pass the splitter 3 and the crystal 4 in a first direction, the splitter 7, a fiber 12 to be inspected, the splitter 7, and the crystal 4 in a direction that is opposite to the first, and the splitter 3 and then reaches a photoelectric receiver 14, thus improving inspection accuracy and stability.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a means for forming a parallel beam of light, a certain series of variable siI frequencies.
n Folge von Modulation), means for modulating the light beam with a modulation of the modulated light beam into or out of the optical fiber to be examined;
nd Auskoppelung) means and means for forming and selecting a modulation signal having a modulation frequency consisting of a constant series (Folge) for driving the modulation means, for testing the optical fiber under optical path length measurement. Regarding equipment.

[Prior art]

Inspecting optical fibers for their length, changes in length or breakage is primarily necessary when the fibers are fixedly strung or laid. Such fibers are then used for information transmission, and by examining the length of fibers inserted into buildings for that purpose,
It is also used to monitor distortions in buildings. Electro-optical methods of measuring the length of optical fibers are either by measuring the transit time of a pulse of light within the fiber or by measuring the phase lag of the modulation after a continuously modulated light beam has passed through the fiber. by measuring. To measure travel time, use Hewlett-Packard's “Time Domain Left Meter” HP 81458 (“Time Dom Left Meter”).
ain Reflectometer”HP8145A
A pulse of light is reflected at the end of the fiber and the round trip time is measured. The accuracy of this device is given as +/-6 m. To measure the phase, use a DisLomat D14L Fir from Bild Heerburg (Switzerland).
ma Wild Heerbrugg (Schwei
z)) is used, in which case the light passes through the fiber only once. That is, two connections are required for introduction and extraction. The measurement accuracy at this time is +/-5 mm.

To measure the speed of light in air or vacuum, 18
46 A, H, L, Fizeau (A, H.

The so-called gear method has been proposed by L. Fizeau. In the phase measurement variant described above, a beam of light is first periodically interrupted by a gearwheel, then sent to a reflector and finally interrupted again periodically by the same gearwheel. If the light rays returning from the rotation speed of the gear are not visible, the travel time of the light ray is determined by the travel time from one tooth of the gear to the gap between the next successive teeth. Modern measuring devices with Fizeau systems use electro-optical crystals as modulators instead of gears (U.S. Pat.
No. 424531 B.L.Hender et al.) (U, S,
-PS 3424531 P, L, Bender e
t al, ), the elliptical polarization of the light beam changes periodically.

A problem is posed when measuring the length of optical fibers using the known Fizeau system. This is because the polarization of the light beam inside the fiber changes due to mechanical stress.

Known systems for temperature compensation of crystal modulators are useless in this case, and fiber vibrations severely disturb the measurements. This is because as the tension changes, the minimum signal to be measured shifts to the maximum signal with interferometric sensitivity. In known phase measuring systems it is not possible to measure in the reflected beam, so two coupling positions are required, each with a separate optical arrangement.

Moreover, such systems have variable deviations due to the coupling between the transmitting and receiving parts, which limit the accuracy to (+/5 mm) as mentioned above. The drawback of the travel time system is that the accuracy is limited to (+/-6 m). The unavoidable reflection of the light beam at the fiber connection locations also becomes a problem if it is indistinguishable from the reflection to be measured.

[Problem to be solved by the invention]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a device for inspecting optical fibers which has improved accuracy and stability and which overcomes the above-mentioned problems of known systems.

[Means to solve the problem]

To solve the above problem, the present invention provides means for forming one parallel light beam, a certain series of modulation frequencies (einer best
immten Folge von Modulati
means for modulating the light beam with a ray of light (on), introducing the modulated light beam into or leading out of the optical fiber to be examined (
the optical fiber under measurement of the optical path length, with means for forming and selecting a modulation signal having a modulation frequency consisting of a constant series (Folge) for driving the modulation means; In the method and apparatus for testing, the modulating means modulates the amplitude of the light beam, and there are means for directing the modulated light beam extracted from the fiber to be tested to the modulating means a second time;
Furthermore, there are detection means for determining its modulation frequency consisting of a certain sequence for the second modulated beam, and when utilizing said sequence between the first and second modulation. The optical path followed by the light beam is an integer multiple of the corresponding modulation wavelength, and the optical path followed by the light beam in the fiber to be examined is 3I from the modulation frequency determined by the detector.
The problem is solved by a device characterized in that there is a means for calculating the problem.

At this time, one end of the fiber to be inspected is equipped with one reflector,
The modulated light beam is then introduced and extracted at the other end and expediently passes through the fiber twice.

[Effect]

The detection means may include a photoelectric receiver and modulation means. i.e. one first polarizing beam splitter, 1
one electro-optic crystal and one second polarizing beam splitter, the parallel beams starting from its source and the first polarizing beam splitter.
a polarizing beam splitter, the electro-optic crystal in a first direction, a second polarizing beam splitter, the fiber to be examined, a second polarizing beam splitter, the electro-optic crystal in the opposite direction to the first direction. in the direction of
It then passes one after another through a first polarizing beam splitter and then reaches a photoelectric receiver.

The modulation means may also include an electro-optically operated directional coupler with four optical connections, each one of which includes means for forming parallel beams, an inspection An optical fiber to be tested, a photoelectric receiver and a non-reflection load are connected, the parallel beam of light being connected to the fiber to be tested or to the non-reflection load, and the fiber to be tested to the photoelectric receiver or to a non-reflection load. Can be connected to a light source.

Other variations of the device of the invention are described in the embodiment section below.

〔Example〕

The invention will be explained in more detail in the following description by means of exemplary embodiments, which are shown diagrammatically in the accompanying drawings.

As shown in FIG. 1, the parallel beam of the l1e-Ne laser 1 reaches a first polarizing beam splitter 3 via a first focusing lens 2. As shown in FIG. The straight-through beam portion becomes plane-polarized and passes through an electro-optic modulator crystal 4, which is present in the wire resonator of a modulator 5. The focal length of the focusing lens 2 is determined in such a way that the diameter of the light beam in the modulator crystal 4 is as small as possible. The now elliptically polarized light beam 6 passes through a second polarizing beam splitter 7 and a second focusing lens 8;
This focusing lens bends the light beam into an intermediate piece of fiber 9.
Focus on the front.

The intermediate piece of fiber 9 can be assembled together with other intermediate pieces of fiber into a multiplex device 10, shown in broken lines, for switching in each case to one of the several fibers to be examined. The intermediate piece 9 is short compared to the fiber to be examined and the other end of the intermediate piece is connected via an adjustable spigot 11 to a fiber 12 to be examined. The diameter of the core of the intermediate piece 9 of fibers must be equal to the diameter of the core of the respective fiber 12 to be examined. A reflector 13 is arranged at the end of the fiber 12 to be examined, which reflects the laser beam back through the fibers 12 and 9. Reflector 1
Reference numeral 3 denotes a glass reflector plated on the front surface which is attached to the end face of the fiber 12 which is either chemically plated at the end of the fiber 12 or polished to a flat surface. On the way back, the laser beam travels again via the beam splitter 7, the modulator crystal 4 and the beam splitter 3, and the correspondingly polarized portion of this beam is directed to the first photoelectron as detection means. It is divided into receivers 14.

The polarization directions of the light beams passing through the polarization beam splitters 3 and 7 are orthogonal to each other. Without the action of the modulator 5, the light beam from the laser 1 would therefore not reach the fiber 12 to be examined at all, but would be transferred to the second photoelectric receiver 1 at the beam splitter 7.
5, the signal from this receiver serves to control and adjust the modulation depth of the beam 6. The modulator crystal 4 is a lithium tantalate crystal, which is stimulated with a modulation signal of varying and adjustable frequency from a microwave source 17 via an amplified transmitter 16. . This is shown diagrammatically in FIG.

If the periodic modulation signal has a frequency of, for example, 1.5 GHz, a light wave of length 20c+n, modulated into elliptically polarized light, is generated every 2/3 nanoseconds, and this light wave is a polarized light beam. It passes through the splitter 7 and travels as an amplitude-modulated light wave towards the fiber 12 to be examined. At the same time, a second light wave, which is in the opposite phase to the light wave that has passed through the beam splitter 7, travels from the splitter 7 towards the control receiver 15. Fiber through which light waves passed9.12
If the length of the modulated wave is an integer multiple of 20 CTI, then the length of the crystal is The laser beam 6 departing at the exit and returning is in the same modulation state at all times. The returning light beam components are then modulated inversely by the crystal 4, so that a minimum brightness occurs in the photoelectric receiver 14.

Since, in the LiTa03 crystal used, the best modulation operating point is highly dependent on temperature, this embodiment provides for modulation and opposite modulation in this same crystal 4. Furthermore, the modulation degree of the output signal of the control receiver 15 is converted into one DC signal and one control amplifier 1
After being amplified at 8, it serves for the temperature-dependent adjustment of the modulator 5. Temperature compensation of the crystal 4 is hereby achieved by regulating the temperature directly or by adjusting the prevoltage by means of an additional electric field at the crystal 4, as shown in FIG. The modulated signal frequently emitted from the amplified transmitter 16 is passed through the input 19 of the modulator 5 to the coupling wire 20.
One branch point is reached, and the coupling coil leads the modulation signal to the modulator crystal 4 at one end of it. The other end of the coil 20 is led via a high-frequency short-circuit capacitor 21 to the input 22 of the modulator 5, to which the DC regulating reserve voltage of the control amplifier 18 is connected.

A constant course of the modulation frequency is set in the microwave source 17 by means of a controller 23 .

A frequency synthesizer with a quartz crystal as the frequency standard is suitable as the microwave source 17. If this is used to change the frequency of the modulator crystal 4, the ratio of the length of the measurement distance to the length of the modulated wave will no longer be an integral multiple, unlike in the case described above. Depending on the modulation frequency, the brightness at the photoelectric receiver 14 exhibits a periodic profile with maximum values and significant minimum values. The output signal of the photoelectric receiver 14 corresponding to such brightness fluctuations is transmitted to one amplifier 24.
and controller 2 via one A/D converter 31
3 and is evaluated by this controller to find the minimum value of the signal.

Therefore, the increasing or decreasing frequency is adjusted one after the other by the controller 23. In this case, each adjusted frequency still has a frequency modulation (frequency shift Wobb).
le), e.g. +/-100KH for IKHz rhythm
The frequency fluctuation of z is added. In this way, the course of the above-mentioned periodic signal at the photoelectric receiver 14 is evaluated by scanning in two detection channels 1 and 2 whose frequencies are adjacent by +/-100 KHz.

In the above case where the length of the measurement distance is an integer multiple of the length of the modulated wave, the travel time T of the light beam from the modulator 5 to the reflector 13 and back is an integer of modulator 1 = 2/3 ns. double g
, that is, T=g−t.

However, at this time, the value of g is still unknown. This ambiguity is removed in a manner known per se as follows. That is, the modulation frequency or modulation period is determined for two or several minimum values of the successive signal curves at the photoelectric receiver 14.

For a multiple g, the minimum value occurs at the modulation period t (g) and for the following nth multiple (g+n) the minimum value occurs at the modulation period t
(g + n), thus in both cases the running time T = g - t (g) = (g + n) · t (g +
n). From this, we can find the multiple g=t by conversion.
(g+n)/(t(g)t(g+n)). At this time, the running time is T = g - t (g), and the measured distance traveled twice is L - (1/2) ・C - T, and the speed of light removal C is known as the fiber to be inspected. depends on the refractive index of The above-mentioned calculations, together with corrections for the refractive index of the fibers, the combined intermediate piece 9 and other corrections, are carried out in a known manner by a microcomputer included in the controller 23.

FIG. 2 shows a further embodiment of the testing device according to the invention, which cooperates with a directional coupler 2'' 525 which can be connected as a modulator. is implemented in the technology of integrated optics in a small space of techniques.In FIG. 2, elements corresponding to FIG. 1 are given the same symbols.

The parallel beam of the laser I reaches via a first input end a directional coupler 25 which can be electrically connected via a control unit 27 . This laser beam is connected to a measuring output 28, to which an intermediate piece of flexible fiber 9 is connected, or via an output 32 to a reflection-free load 29. At this time, depending on the connection state, the light beam returning from the test fiber 12 is transmitted to the output end 30.
or to the photoelectric receiver 14 connected to the output end 2
6.

It is clear that even with this configuration it is possible to modulate the amplitude of the laser beam in the gigahertz range, as shown in the embodiment of FIG. As the electrical configuration shown in FIG. 4 shows, no temperature correction is required in this case. The signal 4 at the output of the receiver 14 reaches the controller 23 via the amplifier 24, where the signal
r) converted into one digital form in converter 31; An evaluation of the signal then takes place as already described in connection with FIGS. 1 and 2.

With the device according to the invention not only lengths of fibers I2 separated by one reflector 13 are examined.

According to the method of the invention, the device described above can also be used, for example, when a fiber is being cut, to evaluate the length of the fiber up to the point where the reflector 13 is not activated and is cut. In such a case, the reflected beam portion is visibly represented on one cathode ray oscilloscope by evaluating the frequency distance of the minimum signal from the output signal of the photoelectric receiver I4 and from that value. It has been shown to be sufficient to evaluate the length of the fiber to the location where the reflection took place.

As described above, the minimum value of the output signal of the photoelectric receiver 14 always appears when the ratio of the length of the measurement distance 5 6 to the length of the modulated wave is an integer. If the length of the modulated wave deviates slightly from this value, the phase difference at the output of the modulating crystal 4 of the departing and returning modulating phases will be:
That is, the difference in the minimum signal of the receiver 14 is greater the longer the length of the fiber 12 to be tested, because the difference in the length of the modulated wave is added along the length of the fiber to be tested. . Therefore, in the case of short fiber lengths to be examined or in the case of reflections at the connection point to the fiber intermediate piece 9 near the modulator 5, the minimum signal will only occur if the length of the modulated wave changes significantly, i.e. as mentioned above. It is possible to measure when the frequency increases by changing the frequency.

However, this also means that when measuring long fibers the unavoidable reflections at the connection points do not cause disturbances, unlike the known systems mentioned at the outset, since such reflections This is because in some cases, it is not detected at all by the detection means.

Further features of the device of the invention are found in the following. Since the light beam is modulated outside the light source 1, it is possible to use light sources that are adapted to different applications in a simple manner. A high power light source for long fibers or a visible light source for conditioning purposes can be easily replaced.

To be able to measure short fibers, for example 3 m long, a modulation frequency of approximately 1.5 GHz is applied without any problems.

Evaluation of the two lowest adjacent signals is sufficient to determine the absolute value of the distance of a fiber of normal length with nXD up to 100 m (where n is the refractive index of the fiber material and D is the geometric length of the fiber). be. The modulator shown in FIG. 1 then uses one narrow band resonator if the fiber to be measured is longer than approximately 5 m. Such modulators require little power and are inexpensive to manufacture.

Embodiments of the invention are as follows.

(1) One end of the fiber (12) to be inspected is connected to one reflective mirror (
13) and the modulated light beam is introduced into or exited from the fiber at the other end and passes through the fiber at 2.
2. The device according to claim 1, characterized in that the device passes through the device at least once.

(2) the detection means include one photoelectric receiver (14) and the modulation means, i.e. one first polarization beam splitter (3), one electro-optic crystal; (
4) and one second polarizing beam splitter (7);
The parallel rays then start from their source (1) and
a polarizing beam splitter (3), an electro-optic crystal (4) in the first
, the second polarizing beam splitter (7), the fiber to be examined (12), the second polarizing beam splitter (7) and the electro-optic crystal (4) in a direction opposite to the first direction. and passing successively through a first polarizing beam splitter (3) and then reaching a photoelectric receiver (14).
The equipment described in section.

(3) A regulating circuit (15, 1
8, 20, 21).

(4) the regulation circuit includes one photoelectric receiver (15);
This photoelectric receiver is stimulated by a portion of the modulated light beam directed from the second polarizing beam splitter (7) and the degree of modulation of the output signal of this photoelectric receiver (15) 4. Device according to claim 3, characterized in that a regulating signal is derived, which signal controls the degree of modulation of the light beam via the temperature of the electro-optic crystal (4).

(5) the regulation circuit includes one photoelectric receiver (15);
This receiver is stimulated by a portion of the modulated light beam guided by the second polarizing beam splitter (7) and from the degree of modulation of the output signal of this photoelectric receiver (15) a regulating signal is generated. This signal is derived from an electro-optic crystal (4
4. Device according to claim 3, characterized in that the degree of modulation of the light beam is controlled via a DC pre-voltage (22) of ).

(6) The detection means include one photoelectric receiver (14) and the modulation means include four optical connections (26, 2
8, 30, 32), each one of these four optical connection ends producing a parallel beam of light. means(
1), optical fiber to be inspected (12), photoelectric receiver (14)
) and one reflection-free load (29), in which the parallel light beams are connected alternately to the fiber to be examined (12) or to the reflection-free load (29) and to the fiber to be examined (12). 2. Device according to claim 1, characterized in that it is connectable alternately to a photoelectric receiver (14) or to a light source (1).

(7) The device according to item 6, wherein the electro-optically controllable directional coupler (25) is completed using integrated optics technology.

(8) The modulated parallel light beam is transmitted by the modulating means (3, 4, 5
, 7, 25) to the fiber (12) to be tested.
8. The device according to claim 1, characterized in that it is introduced or removed by an intermediate piece (9) of fibers having the same diameter but shorter than 12).

(9) Several optical fibers (12) to examine the modulated parallel beams from the modulation means (3, 4, 5, 7, 25)
9. Device according to claim 8, characterized in that it comprises means (10) for switching to one of the following.

00) The reflector (13) at the end of the fiber is a metal-coated surface mirror of glass polished to a flat surface or a chemically silver-plated reflector.
The equipment described in section.

01) Device according to claim 1, characterized in that the constant course of the modulation frequency is in the range of 1.5 GHz.

02) A method for inspecting an optical fiber (12) using the apparatus according to paragraph 2 or 6 above, in which the modulation frequency is constant by the means (17, 23) for generating and selecting the modulation signal. The output signal of the photoelectric receiver (14) of the detection means is visually represented using a cathode ray oscilloscope depending on the modulation frequency, and the extreme values of the output signal are A method characterized in that the length of the fiber to the reflector or to the cutting position is measured from the frequency interval of .

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the optical and mechanical configuration of the device of the invention for testing optical fibers, with one modulator assembly between two polarizing beam splitters. , 2M shows the optical and mechanical configuration of the device according to the invention for testing optical fibers, having one controllable directional coupler as modulator, FIG. 3 as shown in FIG. FIG. 4 shows the electrical configuration of the device shown in FIG. 2, and FIG. 5 shows a preliminary voltage regulating device for the modulator shown in FIG. 3. 1... Means for forming a parallel light beam 3.4.5.7.25... Means for modulating a light beam so that the modulation frequency continues for a fixed length 9.10.11;
. . . Means for introducing the modulated light beam into or out of the optical fiber to be examined 12 . . . Optical fiber
13...Reflector 14.23.24.31...Detection means 15.18.20.21...Regulation circuit 17.23.
. . . Means for forming and selecting a modulation curve with a modulation frequency consisting of a constant course for driving the modulation means 22 . . . DC pre-voltage 26. 28. 30. 3 2. . . 29...Load 3 without reflection 284-

Claims (1)

[Claims] (1) Means of forming one parallel beam of light (1) A constant series of modulation frequencies (einerbestimmtenF
means (3, 4, 5, 7, 25) for modulating the light beam with modulation), introducing the modulated light beam into or leading out of the optical fiber (12) to be examined;
) means (9, 10, 11) and modulation means (3, 4, 5,
7, 25) of the optical fiber under measurement of the optical path length, with means (17, 23) for forming and selecting a modulation signal with a modulation frequency consisting of a constant series (Folge). In the equipment to be inspected, the modulation means (3
, 4, 5, 7, 25) modulating the amplitude of the light beam, the modulated light beam taken out from the fiber to be examined (12) is modulated by the modulating means (3, 4, 5, 7, 25) for a second time. ) are present, in addition detection means (14, 23, 24, 31) for determining its modulation frequency consisting of a constant sequence for the second modulated beam ), and when using said sequence the optical path followed by the light beam between the first and second modulation is an integral multiple of the corresponding modulation wavelength, and the length of the fiber (12) to be examined is means (23) for calculating from the modulation frequency determined by the detectors (14, 23, 24, 31) the optical path traveled by the light ray;
A device characterized by the presence of.